A. Field of the Invention
The invention relates generally to production of simulated watermarks, and more particularly to a process for depositing on uncoated, stationery-grade paper an image similar to a traditional watermark.
B. Description of the Related Art
Watermarking refers to the placement of a faint, permanent image on a sheet of paper. The image can serve to identify the manufacturer of the paper or its user, and/or may contain information relating to date of manufacture or paper composition.
The traditional method of watermarking involves mechanically impressing the design onto the fibers of the paper as it is being manufactured, and while it is still wet. Typically, metal rollers having the design embossed thereon are interposed between the wet paper web emerging from a vessel of aqueous pulp and drying equipment. The rollers are brought into intimate rolling contact with the web, displacing the paper fibers slightly and thereby varying the thickness of the sheet in that area. After drying, the affected area remains permanently translucent when viewed under transmitted light, and the paper web may be cut into individual sheets.
Because this "wet-laid" process is economical only for very large quantities of paper, practitioners have developed a number of processes that simulate the look of a watermark, but which may be applied at low cost to a relatively small number of cut sheets. As far as we are aware, all of these alternative processes involve chemical treatment.
A number of early attempts to produce imitation watermarks involved direct application of colorless resin solutions to dried, finished paper. Penetration of the paper by the resin solution reduced the paper's opacity. Unfortunately, once introduced into the paper, the resins tended to turn yellow in a relatively short period of time, resulting in an unsightly appearance. Other chemical processes involve complex formulations, make use of curing agents which are or give rise to disagreeable vapors or byproducts, require excessive periods of time or special equipment to cure, are difficult to apply, and/or degrade the strength and stability of the paper to which they are applied.
We are also aware of an early attempt to impregnate paper with wax, described in U.S. Pat. No. 1,479,337. In this process, a waxy material is combined with a solvent to produce a liquid or paste, which is applied to the paper from the rollers of a printing press. The solvent carries the wax into the paper fibers, reducing the opacity of the paper. This technique requires use of a full-scale printing press which, in addition to its expense, can create pressure marks on the paper. The method also necessitates manual preparation and maintenance of the waxy liquid material, as well as its repeated application to a printing plate.